High-sensitivity strain probe

ABSTRACT

The present invention is a high-sensitivity strain probe used in high-sensitivity sensor elements of force type. By the use of semiconductor process and wire bonding technology as well as integrated forming method, the fabricated elements include: a probe, a cantilever, a cantilever substrate, resistance materials, and a processing circuit that can be applied to a probe microscope to greatly reduce the number of elements of the scanning probe microscopy. The invention attains the object of lowering the cost and effectively solves the problems of an excessively large signal-to-noise ratio and avoids using the optical elements present in a conventional microscopic probe avoiding various inconveniences and shortcomings of the prior art.

This present application is a divisional of U.S. patent application Ser.No. 08/865,967, filed May 30, 1997 , now U.S. Pat. No. 5,907,095. U.S.patent application Ser. No. 08/865,967 represents a continuation-in-partof U.S. patent application No. 08/664,641 filed on Jun. 17, 1996 which,in its entirety, is incorporated herein by reference.

FIELD OF INVENTION

The present invention relates to a high-sensitivity strain probe able tobe used with high-sensitivity sensor elements of force type. Themicroscopic probe integrated by the use of strain gauge and measuringcircuit, can greatly cut down the number of elements of the scanningprobe microscope, reduce the space of operation, thereby reducing thecost.

BACKGROUND OF THE INVENTION

Various kinds of scanning probe microscopes such as AFM (Atomic ForceMicroscope), MFM (Magnetic Force Microscope), and SNOM (ScanningNear-Field Optical Microscope) are all of the type of microscope whichuses a special microscopic probe to detect certain types of interactionbetween the probe and the surface of a sample, for instance, tunnelingcurrent, atomic force, magnetic force, and scanning near-fieldelectro-magnetic wave. Then, using a piezo-electric ceramic scannerhaving displacements in three axes, allows the probe to scan the surfaceof the sample in front-and-back as well as left-and-right directions. Italso utilizes the capability of minute adjustment in the vertical axisand feedback circuit to maintain its location. The interaction betweenthe probe and the sample during the scanning process makes the distance,which is anywhere between several Å (angstroms) to several hundred Å,relatively constant. One can obtain the interactive action chart of thesample's surface so long as one records the minute-adjusted distance inthe vertical axis for each point on the scanned surface. This data canbe used to derive the surface characteristic of the sample.

The strain probe microscope of the prior art is used to measure theminute action of the microscopic probe by using the reflex refractedangle of a laser. By using a laser to focus on the probe, the beamreflects back to the laser sensor to measure the deformation on theprobe by taking the signal measured from the laser sensor. In order toobtain the optimum amplified signal, one uses the increase of thereflection distance to project and amplify the strain signal of theprobe, hence the range of required space is relatively large. Also, thistype of probe system needs a lot of elements including: the probe, alaser diode, a reflex mirror focusing object lens, a splitted laserinductor, and a signal processing circuit; which have the shortcomingsof being complicated in structure, expensive in terms of opticalelements and not easily used.

Although the disclosure of U.S. Pat. No. 5,386,720 to Toda appears to besimilar to that of the present invention, however, the objective iscompletely different. The major difference is that the wire terminals220, 18, 129, 222, 312, 314, 316 as shown in the Figures and discussedin the specification in the Toda patent are located at the bottom of thecantilever while those of the present invention are on the top of thecantilever. The major disadvantage of placing wire terminals at thebottom is that it is hard to connect wires to them, for example element44 in FIG. 2, since the height of the probe tip is very small (severalμm). The diameter and the connection of the wire 44 will affect thescanning action of the probe tip and the test piece. The presentinvention in contrast changes the location of the wire terminal to thetop of the cantilever. Besides, the Toda patent discloses fabricationonly of the main body of the cantilever and the respective processingcircuit is an exterior type as shown in FIG. 2, FIG. 12, FIG. 16, andFIG. 17, but the volume and space it occupies are all relatively largewhich will result in the situation that the volume of the overall probemicroscope can not be reduced. Besides this disadvantage, since theexterior processing circuits (236, 238, 240) need wire connections,these wire connections will cause the signal interference and noisewhich will further reduce the signal accuracy. Also, the length of thepiezoresistive material becomes longer than what it normally is becauseof the relatively long wire which will result in high noise and arelatively low signal-to-noise ratio. In order to resolve the space andnoise problems, the present invention fabricates the circuit at theterminal of the cantilever which can handle the signal nearby and alsoreduce the volume, thereby, increasing the signal-to-noise ratio andfurther raising the overall measuring accuracy and ability of the probe.

U.S. Pat. No. 5,266,801 makes use of the piezoelectric or piezoresistivematerials to measure the strain on the cantilever, but it still has thenoise problem left irresolvable for the signal along the connectingwire. Also, the exterior transfer and processing circuit occupies arelatively large space and complicates the system structure.

U.S. Pat. No. 5,400,647 measures, by using an Atomic Force Microscope,the transverse force which is related to the magnitude of the frictionalforce. The Atomic Force Microscope makes use of the optical way tomeasure the deformation of the cantilever. Similar to other prior artAtomic Force Microscopes, since they have many optical elements, theirsystem spaces are relatively large, their structures are relativelycomplicated, and the noise problems of their connecting wires stillexist.

U.S. Pat. No. 5,468,959 is a method for measuring the surface not theparticular elements of the probe apparatus itself. Although the patentdoes mention the probe in FIG. 5, however, this probe is not the focusof the patent. This patent mainly describes the use of capacitor andelectro-static force, and the measurement of displacements and externalelectro-static force. This kind of probe is relatively hard to fabricateand its characteristic is still under evaluation.

SUMMARY OF THE INVENTION

In order to overcome the above-mentioned shortcomings, the inventor hasstudied and developed an integrated type of high sensitivity strainprobe which makes use of semiconductor processing and wire bondingtechnology to assemble the following elements: a probe, a cantilever, acantilever substrate, a resistance material, and a processing circuit byemploying the integrated forming mode. This invention can be used withthe existing probe microscope which can greatly reduce the number ofelements to attain the object of reducing the cost and effectivelyresolving the shortcomings of the necessity of using optical elements inthe conventional microscopic probe and the large signal-to-noise ratio.

The main object of the present invention is to provide ahigh-sensitivity strain probe which makes use of resistance materialhaving variable electrical resistance value to directly measure thedeformation of the microscopic probe and further transmit signals to theprocessing circuit at the rear end of the probe to be processed andamplified.

Another object of the present invention is to provide a high-sensitivitystrain probe which can cut down the number of elements, reduce the cost,save time and have high reliability. The invention makes use of theprocessing circuit to process the signal of variable electricalresistance, thereby, it can greatly reduce the effect of noise andinterference.

For these reasons and in order to further explain the stricture andprinciple of the present invention, the inventor herewith presents adetailed and clear illustration together with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS AND INDEX FOR THEIR COMPONENT NUMBERS

(A) The Drawings

FIG. 1 is the schematic diagram of the outward appearance of the presentinvention.

FIG. 2a through FIG. 2m are the schematic process diagrams of thepresent invention.

FIG. 3 shows the geometrical relation of the V-shape pit in thecantilever substrate.

FIG. 4 is the block diagram of another embodiment of the probe typemicroscope.

FIG. 5 is the structural diagram of the strain gauge of FIG. 4.

FIG. 6 is a plan view of the bottom of the strain gauge of FIG. 4 whenpiezo-electric material is placed there.

FIGS. 7a and 7b show examples of possible patterns for thepiezo-electric material.

FIG. 8 is the processing circuit diagram for the strain gauge of FIG. 5.

FIGS. 9a and 9b show other embodiments for the structure of theresistance material.

(B) INDEX OF THEIR COMPONENT NUMBERS

1 Probe

2 Cantilever

3 Resistance material (polysilicon resistance layer)

4 Processing circuit

6 Connecting wire

7 Cantilever substrate

8 Pad for processing circuit

9 Silicon dioxide

10 Positive Photoresist

11 Probe main body

12 Polysilicon

13 Silicon Nitride

30 Piezo-electric driving device

40 Calculation and control unit

50 Analog/digital signal converter

51 Output pad

52 Pad for resistance material

53 Power supply pad for processing circuit

60 Cantilever

61 Probe

62 Processing Circuit

65 Piezo-electric material

67 Pad for piezo-electric material

70 Cantilever

72 Resistance material

75 Input voltage pad

76 Output voltage pad

601 Bridge resistance

621 Wire connecting terminal

622 Resistance/voltage signal converter

623 Temperature compensator

624 Differential amplifier

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIG. 1, the present invention makes use of the semiconductorprocessing technology to fabricate the probe 1 and resistance material 3on the cantilever 2. Due to the limitation of the moving space for theprobe 1 and wiring work, the present invention fabricates all the wireterminals on the opposite side of the surface of the cantilever fromwhere the probe is located. The main function of the probe is to produceinteractive action when it touches the sample to be tested, thereby togenerate deformation of the cantilever 2. A processing circuit 4 is setup on the cantilever substrate 7 and is connected to the resistancematerial 32 by using a connecting wire 6 connected between the pad forthe processing circuit 8 and the pad for the resistance material 52. Thefunction of processing circuit 4 is to provide energy conversion fromresistance to voltage using a Wheatstone Bridge, an instrumentamplifier, a nonlinear compensatory circuit and a filter. Afteranalog/digital signal conversion, calculation, and processing by theprocessing circuit, the signals are sent to the piezoelectric drivingdevice in order to perform accurate 3-axis displacement. The signals,after being processed, are sent out through the output pad 51 of thehigh sensitivity strain probe. The power supply pad 53 for theprocessing circuit is used to connect to an outside power source. Thedevice advantages are as follows:

1. The components used are simple, which makes the price low.

2. Signal amplification does not make use of the reflecting space, whicheffectively saves space.

3. The device is very convenient to operate, even non-professionalpersons can attain the same requirements.

4. The distance does not need to be adjusted, which effectively shortensthe time required for testing.

5. The device employs integrated forming of semiconductor process, whichlowers the noise that interferes with the circuits.

FIG. 2a through FIG. 2m are the schematic diagrams of the integratedforming process. The general process steps are as follows:

a. The cantilever substrate 7 is fabricated from p-type silicon wafers,4 inches long, 450 μm thick, with both sides polished and is depositedwith masking material like silicon dioxide (SiO₂) as shown in FIG. 2a.

b. A layer of positive photoresist 10 is applied on the top layer of thesilicon dioxide to serve as the masking material. The masking materialis patterned by the use of buffered oxide etchant (BOE) at a etchingrate of around 800 Å per minute. The resulting mask shown in FIG. 2bserves as a mask for anisotropical etching in the next step.

c. A mixture of 25% potassium hydroxide (KOH) is used to perform wetetching at 75° C. to form the contour shape of the bottom surface of thecantilever in the cantilever substrate as shown in FIG. 2c. Hydrofluoricacid is then applied to remove the masking material on the surface ofthe cantilever substrate.

d. One uses the same process to deposit a masking ma:erial of either athermal oxide or silicon dioxide 9 and define the cross-sectionaltriangle of the pit. The pit is etched into the cantilever substrate asa V-shaped opening as shown in FIG. 3 to serve as a mold for the probetip which preferably is conical but may have a pyramidical shape.Potassium hydroxide (KOH) is used to carry out wet etching to form thepit as shown in FIG. 2d. In the preferred embodiment, the inclined sidesof the pit have an angle of 54.7 degrees. The width (w) of the openingto the pit has a geometrical relation to the height (h) of the pit. Thepreferred embodiment has a height of 3.5 μm when a width of 5 μm is usedfor the opening. After the pit is formed, the mask is removed from thesubstrate by the etchant BOE.

e. A 2 μm thick film of thermal oxide is deposited on the silicon waferat 1100° C. for 10 hours. The silicon wafer is annealed at 1100° C. for1 hour to release the residual stress. The film is patterned and alignedto define the overall main body of the cantilever and the probe. Thensilicon dioxide is deposited in the pit in order to form the tip of theprobe as shown in FIG. 2e.

f. The polysilicon 12 possesses the advantage of having a gauge factorgreater than other materials generally used as a piezoelectric resistor.A 1500 Å thick polysilicon film of low pressure chemical vapordeposition (LPCVD) is deposited at 620° C. to be used as the materialfor the strain gauge circuit (resistance material) 12 as shown in FIG.2f.

g. In order to make the polysilicon electrically conductive, the POCL₃blanket n+ diffusion is operated at 950° C. for 30 minutes as shown inFIG. 2f. The film then is patterned and aligned as a mask to define theresistance material 3 as shown in FIG. 2g. An etchant solvent mixturewith HF:HNO₃ :CH₃ COOH=1:26:33 is used to etch at a rate of 1500 Å perminute.

h. In order to protect the resistance material during aluminumpatterning and back etching later, a 1000 Å thick LPCVD film of siliconnitride (Si₃ N₄) 13 is deposited at 750° C. as shown in FIG. 2h. Thesilicon nitride film also isolates the polysilicon from the aluminum.

i. The film is patterned and aligned to generate the hole openings toform the contact pads. The positive photoresist is first spun coated onthe patterned surface. The photoresist is patterned and aligned in thesame way as the S₃ N₄ film. That is the two successive processes use thesame mask as shown in FIG. 2i.

j. A 5000 Å thick of aluminum film is evaporated by the thermal coater.The lift-off technique is employed to define the aluminum contact pads,i.e., the output pad 51 and pad for resistance material 52. The lift-offtechnique is accomplished by lifting off the aluminum film with thephotoresist stripping. The result is shown in FIG. 2j.

k. Before the wafer is etched from the back with KOH, a S₃ N₄ film toprotect the aluminum contact pads and the main structure is formed asshown in FIG. 2k. Although the etching rate of around 60 Å per minute ofoxide is extremely slower than the etching rate of around 1 μm perminute of silicon in 25% concentration of KOH mixture at 75° C., theoxide film exposed to the etchant also needs to be protected because ofthe long etching time. The wafer is then immersed in a 25% concentrationof KOH mixture at 75° C. After approximately 7.5 hours, the siliconbelow the bottom plane of the cantilever is removed by the wet etchingwhen the etching is stopped at the level of the oxide film. Theprotective S₃ N₄ film is removed by reactive ion etching (RIE) which isoperated at 100 watts by a mixed gas with CF₄ :O₂ =40:5 (the etchingrate of S₃ N₄ is around 700 Å per minute), and the device is shown inFIG. 21.

1. Finally, the processing circuit is applied and connecting wires 6 areconnected to each pad as shown in FIGS. 1 and 2m.

FIG. 4 is a block diagram of another strain probe of the presentinvention. The cantilever structure uses the process technology of asemiconductor to provide a surface with piezo-electric material on thecantilever 60 to provide the deformation signal when the cantilever andprobe are subjected to a force. The resistance variation signal istransferred through wiring on the top of the cantilever 60 to provide asignal to the processing circuit 62. After processing by theanalog/digital signal transducer 50 and the calculation and control unit40, a signal is sent to the piezo-electric driving device 30 to allowfor accurate 3-axes movement.

FIG. 5 is the structural diagram of the present invention. The overallprofile shows a "T" shape with a surface having piezo-electric material(a bridge resistance 601 consisting of R1, R2, R3 and R4) that may beplaced on either or both the top and bottom (shown in FIG. 6) of thecantilever 60. FIGS. 7a and 7b show examples of patterns that thepiezo-electric material 65 may be etched as with attached pads 67 forthe piezo-electric material. When the probe is subjected to force, thedeformation signal can be converted into a signal representing thevariation in the electrical resistance to provide the processing circuitwith an amplified signal. The signal then is filtered and amplifiedthrough the processing circuit 62 to convert the signal into a voltagefor processing by the analog/digital signal converter 50 and thecalculation and control unit 40. After processing, the signal is sent tothe piezo-electric driving device 30.

A piezoelectric system is a scanning system having a piezo-electrictransistor 3-axes positioner (not shown in the Figures) which mainlyprovides the test piece with the functions of scanning and up-and-downmotion. The piezo-electric driver is the power driver of thepiezo-electric system. It receives the output signal from thecontroller, then the signal is amplified to the voltage value requiredby the piezo-electric transistor, which generally needs high voltage,through a power amplifier. FIG. 8 shows the processing circuit 62 whichincludes a resistant/voltage signal converter 622, a temperaturecompensator 623, and a differential amplifier 624. Among them, the inputterminal of the power source is V_(dd), the output terminal of thesignal is V_(out), the resistance/voltage signal converter 622 isconstructed with an electrical bridge to perform the function oftemperature compensation, while the temperature compensator 623 and thedifferential amplifier 624 are constructed with a p-channel andn-channel of metal oxide semiconductor (MOS).

In another embodiment the resistance material 72 on the cantilever 70 ismade with a Wheatstone bridge circuit with resistors R5, R6, R7, and R8as shown in FIGS. 9a and 9b. Input voltage pads 75 and output voltagepads 76 for the resistance material provide connection points to theprocessing circuit (not shown). A Wheatstone bridge not only has theadvantages of increasing output voltage, but also it can compensatetemperature. Two cases designed by the principle of a bridge circuit areshown in FIGS. 9a and 9b, pads 75 provides the input voltage and pads 76are for the output voltage. FIG. 9a is the basic design which only hasone active resistor. Once the temperature varies, the FIG. 9a circuithas difficulties compensating for temperature. In FIG. 9b, R5 is anactive resistor and R6, R7, and R8 are fixed value resistors. The designcan compensate temperature because R5, R6, R7, R8 nearby suffer the samesituation so that the term of temperature effect cancels out. Actually,if all four resistors are active, the voltage output will be larger thanthe value from the FIG. 9b circuit. However, according to our analysis,we know that when the resistor layer is added to the cantilever, thestrain will be decreased because the thickness of resistors constrainsthe bending of the cantilever. Based on this reason, there must be acompromise in temperature effect and the number of active resistors.

From the foregoing statements, the present invention, because of itsspecial fabrication process, is a high-sensitivity strain probe withhigh reliability which is a design having great practical value that cancut down the number of elements required, lower the cost and save time.By the use of solid forming type of semiconductor design, it can protectits accuracy from being affected by circuit errors, noise and externalinterference, thereby, greatly raising the system reliability withoutneeding to adjust a laser beam which can save time and is veryconvenient.

To summarize the above-mentioned statement, the high-sensitivity strainprobe of the present invention can provide an effective method, in thelight of the shortcomings of prior art probe devices, such thattechnicians can enjoy its convenience and practical capabilities. It isthe inventor's belief that the present invention will be very beneficialfor industry.

Although the present invention has been illustrated and describedpreviously with reference to the preferred embodiments thereof, itshould be appreciated that it is in no way limited to the details ofsuch embodiments, but is capable of numerous modifications within thescope of the appended claims.

What is claimed is:
 1. A method of fabrication of a high-sensitivitystrain probe comprising the following steps:providing a cantileversubstrate, shaping the cantilever substrate, forming a pit in thecantilever substrate so as to provide a mold for a probe, forming theprobe in the pit, forming a cantilever on the cantilever substrate andthe probe such that the probe depends from the cantilever, forming aresistance material on the cantilever, forming a plurality of pads onthe cantilever, removing a portion of the cantilever substrate up to aplane of the cantilever, depositing a processing circuit on thecantilever, and providing wires connecting the plurality of pads,processing circuit and resistance material.
 2. The method of fabricationof a high-sensitivity strain probe as defined in claim 1, wherein thecantilever substrate is formed from silicon wafers.
 3. The method offabrication of a high-sensitivity strain probe as defined in claim 1,wherein the step of shaping the cantilever substrate includes thefollowing steps:depositing a masking material on a surface of thecantilever substrate, etching the surface of the cantilever substratewith a mixture containing potassium hydroxide, and removing the maskingmaterial with hydrofluoric acid.
 4. The method of fabrication of ahigh-sensitivity strain probe as defined in claim 1, wherein the step offorming a resistance material includes the following steps:depositing alayer of polysilicon on the cantilever, making the polysiliconelectrically conductive, patterning and aligning the polysilicon, andetching with a solvent.
 5. The method of fabrication of ahigh-sensitivity strain probe as defined in claim 1, wherein the step offorming a plurality of pads includes the following steps:depositing asilicon nitride film over a top surface of the cantilever and theresistance material, patterning and aligning the silicon nitride film,etching to form openings for the contact pads, spin coating aphotoresist, patterning and aligning the photoresist identical to thesilicon nitride film, evaporating an aluminum film, and lifting off aportion of the aluminum film on the photoresist.
 6. The method offabrication of a high-sensitivity strain probe as defined in claim 1,wherein the step of removing a portion of the cantilever substrateincludes the following steps:depositing a silicon nitride film over theplurality of pads, the resistance material, and the cantilever, etchingwith a mixture containing potassium hydroxide, and removing the siliconnitride film with reactive ion etching.